Breast cancer is a leading killer of American women. However, very significant technical improvements in the sensitivity of mammography have been made over the past decade leading to much earlier detection of cancers in many patients. Early diagnosis and treatment results in dramatically improved cure rates. With the improved radiographic resolution which is available in current mammographic studies, radiologists are detecting a greater number of suspicious densities, microcalcifications and other tissue distortions that may represent an early cancer. Biopsy of these suspicious findings is very frequently recommended to determine whether or not the patient has a malignancy.
Customary surgical biopsy is an invasive procedure that requires hospitalization, general anesthesia, a surgical incision and the removal of one or more moderately large tissue specimens to be certain that the area in question is removed. Within the last 5 years, a method of tissue sampling has been developed, wherein a needle or canula is "fired" or injected into and through a suspicious lesion by a spring loaded biopsy device or "gun". In this method, known as stereotactic guided needle biopsy, the sampling needle is guided by preliminary stereotactic radiographs from which the exact location and depth of the lesion is calculated. This information is programmed into the biopsy device so that the sampling needle can be guided to and traverse the area to be sampled. Accuracy of needle placement is plus or minus 2 to 5 mm.
As mentioned above, the sampling needle must pass through the suspicious lesion. The minimum needle "throw" or excursion to obtain a satisfactory core from the lesion is about two centimeters. Since the minimum depth of a lesion to be biopsied in this procedure must be at least 1.0 to 1.5 cm, the actual needle movement is at least 3.5 cm through tissue. For this reason, the breast cannot be reduced in thickness to less than 3.5 cm in order to perform the biopsy by this method.
Newer needles for use in stereotactic guided needle biopsy have a trough about 2.0 cm in length near the tip. When the needle is inserted into a lesion, tissue from the lesion is drawn by suction into the trough and then excised by an inner coring sleeve. Needles of this design are rapidly gaining favor since they provide larger tissue samples which provides more accurate tissue analysis. However, these newer needles must also traverse the length of the tissue to be sampled, and the required amount of breast tissue traversed remains at no less than 3.5 cm.
Known apparatus for performing stereotactic guided needle biopsy is illustrated schematically in FIGS. 1 and 2. The apparatus, generally designated 10, includes a table 12 which can be elevated. The patient lies face down on a mattress 14 supported on the table. and the patient's breast 16 containing a lesion to be biopsied, such as the lesion 17, is suspended through a circular opening 18 approximately 20 cm. in diameter. A fixed, rigid plate 20 is position at one end of the opening 18 adjacent one side of the breast 16. A radiographic recording mechanism 22, either mammography film or an electronic digital recording screen, is located behind the fixed plate. On the other side of the breast, there is a movable, mechanically driven, translucent pressure plate 24 with a window opening 26. A biopsy needle 28, which is mounted in a spring loaded "gun" or injector 29, is aligned with the window opening 26, and a mammography xray tube 30 is positioned proximal to the compression plate.
In practice, the pressure plate is moved toward the fixed plate in the direction indicated by arrow A to moderately press or squeeze the breast between the plates 20 and 24. After initial radiographic images are taken to identify the lesion 17, the window 26 is positioned directly over the lesion. Two stereotactic views are then obtained by angling the xray tube 15 degrees to either side of a central line aimed at the lesion. The figures are programmed into the apparatus to determine the depth of the lesion within the breast.
Under sterile conditions and with local skin anesthesia, the biopsy needle 28 is then inserted into the breast just proximal to the target lesion 17. Accuracy of the needle placement is then assessed by means of another pair of stereo images. If the operator confirms that the area to be biopsied is directly opposite the needle tip, the spring loaded injector then "fires" or injects the needle at a high speed for a distance of about two centimeters so that the needle traverses the length of the lesion. A third set of stereotactic views is then taken. If the needle placement is satisfactory, sampling then proceeds.
As a woman ages, the supportive fibers of the breast lose their elasticity and the fibers elongate. These changes also occur in the skin envelope encompassing the breast. Although there are variations from patient to patient, some breasts become elongated, flaccid, and pendulous. When such a breast is initially pressed between the plates 20 and 24 during the first steps of a stereotactic needle biopsy, the total thickness of the breast can be as little as two centimeters. If the needle is injected in such a case, the 2 cm excursion of the needle will very likely cause the needle to pass through the back side of the breast, impinge on the fixed plate 20, break or penetrate the plate and impale the patient's breast. Accordingly, in cases where the patient's breast, after it is pressed between the plates 20 and 24, is too thin for stereotactic biopsy, i.e., the breast does not have a thickness greater than about 3.5 cm, stereotactic needle biopsy must be abandoned and the biopsy performed with a full surgical approach.
Human tissue is not compressible, i.e., there is no reduction in volume when human tissue and, more particularly, breast tissue is pressed or squeezed. Moreover, the skin has a very limited ability to stretch. Accordingly, when a breast is pressed or squeezed there is no change in the total volume of breast tissue. Instead, the tissue is displaced, flattened and altered in shape up to the limits permitted by the supportive fibers of the breast and the confines of the skin envelope.
When the patient's breast is pressed between the fixed plate 20 and the pressure plate 24 as shown in FIG. 2, the breast generally has the configuration of a half cylinder. The volume, V, of the tissue may be roughly computed by the following mathematical formula: ##EQU1## Where .pi. is 3.14, r is the radius of the tissue cylinder, and h is the height or thickness of the cylinder.
If the radius r of the patient's breast is reduced, which is accomplished by reducing the area of the breast in contact with the pressure plate, then h or the thickness of the breast must increase in order to reflect the same volume of tissue. This reduction in the breast area in contact with the compression plate may be accomplished by restricting the displacement of the breast interiorly, medially and laterally as it is compressed.